Disk drives typically use heads residing on sliders to read from and write to the magnetic media. Read and write transducers residing in the head are flown at a small, controlled spacing above the magnetic medium during read and write operations. Although generally desired to operate in close proximity to but not touching the disk, the head may also contact the media. This prolonged contact, for example on the order of tens of microseconds or more, is known as touchdown. For example, heads typically use a thermal actuator that generates heat to control the head-media spacing. Heat generated by the thermal actuator causes local thermal expansion of the head, which locally reduces the spacing between the head and magnetic media. The thermal actuator can be driven to induce sufficient heating for contact between the head and media, or touchdown. This touchdown is intentional and is generally performed on each drive during initial drive calibration. Touchdown may also occur at other times during disk drive operation, for example due to changes in environmental conditions, operation of the disk drive outside of desired parameters, or contamination to the head that causes the prolonged contact described above.
Touchdown is detected in the drive operation as well as in testing. Conventional touchdown detection may be performed using a variety of techniques. For example, touchdown sensors consisting of a single layer of NiFe has been used. NiFe typically has a relatively large temperature coefficient of resistivity (TCR). The change in resistivity of a NiFe film with temperature may thus be relatively high. As the disk drive experiences touchdown, the temperature of the NiFe sensor changes. For example, a 0.1 degree Celsius change in temperature may abruptly occur upon touchdown. The change in temperature changes the resistivity of the NiFe sensor. Using this jump in resistivity of the NiFe sensor, touchdown might be detected.
Although such conventional methods for detecting touchdown function, there are drawbacks. The NiFe sensor may erase the media. The NiFe layer in the touchdown sensor may have multiple domains. Under the influence of a small external field, the domain wall(s) between domains may move. The movement of the domain wall or domain wall nucleation results in stray fields. These stray fields may result in unexpected erasure of the media. Further, the NiFe layer in the touchdown sensor is magnetic. As a result, the NiFe layer may have stray fields at its ends. The stray fields may result in adjacent track erasure, or wide area track erasure (WATER), which is undesirable.
Accordingly, what is needed is a system and method for providing improved touchdown detection.